Precision resistors by definition require a defined precision resistance. However, without special treatment, the resistance of the resistor varies substantially with environmental parameters. Specifically, when electrical power is applied to a resistor such as a Silicon Chromium (SiCr) Thin Film resistor, the heat generated by the power passing through the resistor substantially decreases the resistance of the resistor. FIG. 1 illustrates a resistance curve of an untrimmed resistor as a function of power applied to the resistor. As shown in FIG. 1, the resistance may decrease substantially as the power applied to the untrimmed resistor increases. This is due to an increase in dissipated power in the resistor, that results in an rise in the resistors junction temperature. As temperature increases, there is an increase in free electron density in the heated resistor and thus the resistance decrease. The amount of resistance change as a function of power may be characterized by a power coefficient of resistance (PCR) which may be defined as PCR=ΔR/ΔP, where ΔP is the power change and ΔR is the resistance change. PCR corresponds to the slope of the resistance curve. A related parameter of the resistor is the temperature coefficient of resistance (TCR) which may be defined as TCR=ΔR/ΔT, where ΔT is the temperature change and ΔR is the resistance change.
In practice, a resistor is usually designed to have an absolute resistance value R which is ideally substantially constant with respect to the changes of power that passes through the resistor or the temperature on the resistor body. Thus, it is not desirable to have a resistor whose resistance varies over the power applied to it. One way to create a robust resistor that has no or very little resistance variability over power variations is to apply special treatments such as resistor trimming to the resistor before deployment. Resistor trimming is a process that stabilizes the resistance value of a resistor within a precision range. The resistance of a resistor may be trimmed in different ways. For example, current art may include current trim (ITrim), laser trim, or mechanical trim. Each of these trimming methods may have their respective characteristics. However, after trimming, the resistance of the resistor may stay within a certain range of an absolute resistance. FIG. 2 illustrates a comparison of the resistance curves of a resistor before and after trimming using the Itrim method. FIG. 2 shows that the resistance-over-power curve of a trimmed resistor may not reduce as dramatically as an untrimmed resistor.
Current art commonly employs a single particular trimming approach to trim the resistor. Because of the limitation of each particular trimming approach, it is difficult to achieve a high precision resistor using the current art. Therefore, there is a need to improve the current art to achieve high precision resistors.